1. Field of the Invention
The present invention generally relates to the field of wireless communication systems. More specifically, the invention relates to mixed voice and data transmission for wideband code division multiple access communication systems.
2. Related Art
In a wideband code division multiple access (“WCDMA”) communication system, transmission is provided for voice communication and data communication simultaneously by transmitting voice and data signals across one or more communication channels. Voice communication and data communication differ in their signal characteristics. For example, voice communications tend to comprise signals which have relatively low information rates, i.e. bit rates, and which are relatively continuous in the sense that the bit rates vary over a limited range of values. Data communications, on the other hand, tend to comprise signals which have a relatively high bit rate and occur discontinuously in time, i.e., in short “bursts” of data at high bit rates separated by periods of relative inactivity, or quiet, in which the data bit rate is low. In order to provide efficient transmission for these different types of communication signals, different communication channels can be provided for each type of communication signal. One way to provide different communication channels is to specify a different part of the frequency spectrum, i.e. a different “band” of frequencies or frequency band, for each different channel.
Efficient use of limited bandwidth of the communication channels is improved by providing a “shared channel” for multiple users to transmit data signals and also providing a “dedicated channel” for transmitting control signals regarding the data information on the shared channel and also for transmitting voice signals. The dedicated channel is always “on,” i.e. transmitting voice and control information at a relatively low bit rate. In this way, a data user can “listen” to the dedicated channel, or be signaled over it, to find out whether to receive data from the shared channel. Thus, the different types of communication signals can be segregated into different channels. By segregating the data signals into a shared channel, channel resources in the dedicated channel can be allocated more efficiently. For example, if data communication signals were sent over the dedicated channel various limited channel resources, such as orthogonal spreading codes—further discussed below—would be allocated to the relatively quiet periods as well as to the bursts. Such allocation is wasteful compared to the allocation of channel resources to communications which occur at a relatively constant rate, such as control signals and voice communications. Thus it is more efficient to restrict the dedicated channel to those communications which occur in a limited range of bit rates, such as control signals and voice communications.
Furthermore, by segregating the data signals of multiple users into a shared channel, channel resources in the shared channel also can be allocated more efficiently. For example, channel resources, such as orthogonal spreading codes, can be allocated in a way that takes advantage of the bursts in the data signals to make transmission of the signals more efficient. One way to allocate orthogonal spreading codes to make signal transmission more efficient is to vary the length of the codes and thereby vary the amount of spreading which is performed.
FIG. 1 illustrates an example of a timing relation between communication channels. FIG. 1 shows shared channel 102, for multiple user transmission of data signals, in schematic form as a sequence of data frames. Data frame 104, for example, contains data information from one specific user. Shared channel 102 is also referred to as a “physical downlink shared channel” (PDSCH) as indicated in FIG. 1. FIG. 1 also shows dedicated channel 106, also referred to as a “dedicated physical channel” (DPCH), for transmitting control signals regarding the data information on the shared channel and also for transmitting voice signals. Channel 106 is also shown in schematic form as a sequence of “DPCH” frames. DPCH frame 108, for example, contains control signal and other information regarding the data information in associated data frame 104.
Each DPCH frame is formatted as a series of “slots.” FIG. 1 shows DPCH frame 108, for example, in expanded form as series 108 of several slots. Each slot in example series 108 can further be shown in expanded form. For example, the i'th slot 110 of series 108 where i ranges between 1 and 15 corresponding to the number of slots in series 108, is shown in expanded form as slot 110. Slot 110 contains information comprising voice and possibly other information, indicated as “voice information” in slot 110; power control information, indicated as “power control” in slot 110; pilot information, indicated as “pilot” in slot 110; and TFCI (“transport format combination indicator”) 112, indicated as “TFCI” in slot 110. Thus each DPCH frame, and in particular, DPCH frame 108, contains TFCI information, such as TFCI 112, which is spread throughout each DPCH frame in slots, such as slot 110.
As stated above, one way to allocate orthogonal spreading codes to make signal transmission more efficient is to vary the amount of spreading which is performed, i.e. change the spreading factor from frame to frame as data frames are transmitted on shared channel 102. Continuing with FIG. 1, the TFCI in each slot, such as TFCI 112 in slot 110, includes spreading factor information for associated data frame 104. Thus, data frame 104 cannot be de-spread using orthogonal codes until the spreading factor information has been received from a TFCI, that is, until the complete associated DPCH frame 108 has been received. FIG. 1 shows that the beginning of data frame 104, marked by dashed line 114, is received before the end of associated DPCH frame 108, marked by dashed line 118. Thus, as shown in FIG. 1, DPCH frame 108 and associated data frame 104 are transmitted approximately simultaneously.
FIG. 2 illustrates an example of how voice, control, and data information can be received in a WCDMA or spread spectrum communications system. Exemplary system 200 shown in FIG. 2 constitutes part of a receiver which may generally reside in a subscriber unit when the communication is taking place in a downlink channel. In exemplary system 200 shown in FIG. 2, receive FIR 252 receives signal 251, containing voice and control information and data. Signal 251 is transmitted to receive FIR 252 across communication channels, which can be shared channel 102 and dedicated channel 106 shown in FIG. 1.
Continuing with FIG. 2, shared channel data from receive FIR 252 is provided to chip level buffer 254. Chip level buffer 254 is used to store the shared channel data until the TFCI information, which includes the spreading factor information, is received from the associated DPCH frame that is transmitted approximately simultaneously on the dedicated channel. As a result of spreading, each data symbol is represented by several bits of signal information, called chips, in the transmitted signal. Spreading the data symbols into chips, using various forms of coding, for example, can spread the data by a factor of 1,000 or more, i.e., each data symbol is represented by 1,000 chips in the transmitted signal. Thus, the amount of storage needed to store the data as chips in chip level buffer 254 can typically be on the order of one thousand times the amount of storage needed to store the original data as symbols before coding, i.e., at the symbol level, before transmission.
Only after TFCI information has been completely received, data chips are passed from chip level buffer 254 to de-spreader 256 and Walsh de-cover 258. De-spreader 256 “de-spreads” the PN (pseudo-noise) spreading provided at the transmitter. Output of de-spreader 256 is passed to Walsh de-cover 258. Walsh de-cover 258 “undoes” the orthogonal spreading applied to the data at the transmitter. In this example, the particular type of orthogonal spreading is called “Walsh covering.” Thus, Walsh de-cover 258 restores the data to its condition at the transmitter before orthogonal spreading, i.e., Walsh covering, was applied. Walsh de-cover 258 restores the data by applying the inverse function of the original Walsh function spreading. Walsh de-cover 258 uses spreading factor 255 to determine the correct inverse function to use to de-spread, or de-cover, the data chips. Spreading factor 255 is fed to Walsh de-cover 258 as soon as it is available at the receiver. Chip level buffer 254 provides a time delay to allow input of spreading factor 255 to Walsh de-cover 258 before output of de-spreader 256 is passed to Walsh de-cover 258.
The output of Walsh de-cover 258, which is a sequence of symbols, is then passed to de-scrambler 264. De-scrambler 264 de-scrambles the sequence of symbols by inverting the operations used to scramble them. The de-scrambled symbol sequence is passed on to de-interleaver 266. De-interleaver 266 undoes the interleaving performed at the transmitter on code symbol sequences. Thus, the output of de-interleaver 266 is code symbol sequence 267 comprising an encoded sequence of symbols. Code symbol sequence 267 should ideally be the same as the original code symbol sequence that was interleaved at the transmitter. The output of de-interleaver 266, i.e. code symbol sequence 267, is passed on to decoder 268. The output of decoder 268 is user information sequence 269, which ideally is identical to the original user information sequence that was transmitted. Thus, FIG. 2 illustrates an example of reception and recovery of a user's information signal, which may include voice, control, and data information, using both dedicated and shared communication channels.
Because the control information is transmitted simultaneously with the data information, as discussed above in relation to FIG. 1, the data information must be stored at the receiver in its spread form, i.e., as chips, until the control information necessary to de-spread the chips into symbols is received. The de-spreading must be accomplished before further processing can commence for recovering the data in its original form. Thus, according to the techniques described in relation to FIG. 1 and FIG. 2, there is an inherent processing delay at the receiver. Further, because the data information must be stored at the chip level, i.e., as chips, a relatively large buffer must be used. As noted above, a chip level buffer can typically be 1,000 times larger than a symbol level buffer which stores the same data information. The downlink receiver for WCDMA systems is typically in the terminal unit, which is typically a small portable unit that is constrained to be efficient and to have low power consumption. Thus, extra buffer and processing power requirements for the terminal unit constitute a severe disadvantage.
Thus, there is a need in the art for transmitting voice, control, and data information without causing a processing delay at the receiver. There is also a need in the art for transmitting voice, control, and data information which does not require excessive storage space at the receiver.